1. Field of the Invention
The present invention relates to a filter and a radio communication apparatus using the same.
2. Description of the Related Art
Generally, a bandpass filter in a radio communication apparatus comprises a plurality of resonators arranged in cascade. Each resonator includes an inductor and a capacitor, and further contains a resistor when an effect of a loss is considered. In the filter of this type, it is possible to determine a passband frequency range and an attenuation amount of stopband by determining an inter-resonator coupling coefficient representing a coupling quantity between adjacent resonators and a value of an external Q factor representing a quantity to excite the resonators in the input and output ports thereof properly.
A filter using a microstrip line type resonator is known as such a filter. It is desired to miniaturize the filter in the radio communication apparatus. In the filter using the microstrip line type resonator, various approaches for miniaturization of the filter are proposed.
For example, U.S. Pat. No. 6,633,208B2, FIG. 11, discloses a filter approximating a plurality of half-wavelength resonators each having a configuration such that a narrow meander line is connected between ends of a pair of broad lines. The narrow meander line is arranged between the broad lines.
U.S. Pat. No. 5,055,809 (FIGS. 7 to 15) discloses a filter approximating a plurality of half-wavelength resonators each having a configuration such that a narrow line is connected between the ends of a pair of broad lines. The narrow lines are arranged on a side opposite to the open ends of the broad lines. Further, the width of the region on which a pair of broad lines are formed equals that of the region on which the narrow lines are formed.
It is preferable that the coupling between the adjacent resonators is capacitive coupling for the filter to be miniaturized. However, according to the filter of U.S. Pat. No. 6,633,208B2, a pair of broad lines and a pair of narrow lines approximate to each other with respect to an arrangement direction of the resonators, so that the full length of the filter in the arrangement direction of the resonators increases resulting in limiting miniaturization of the filter.
In the case of the filter of U.S. Pat. No. 5,055,809, when the adjacent resonators are closed to each other for miniaturization, the capacitive coupling occurring between the adjacent resonators due to a pair of broad lines has substantially the same value as the inductive coupling occurring due to the narrow lines. As a result, the capacitive coupling and the inductive coupling are cancelled to each other so that the deviation of coupling coefficient due to the dimensional accuracy increases. The deviation of coupling coefficient deteriorates the filter characteristic, resulting in increasing fluctuation of filter characteristic at the time of production of the filter.